The present invention is related to fiber optic technology, and more particularly, to fiber optic couplers for single mode optic fibers.
Modern fiber optic networks generally use single mode optic fibers to transmit light signals having a particular wavelength. Such networks typically have numerous couplers by which a signal on one fiber is distributed to two or more fibers. In a typical coupler, a single input fiber joins two output fibers to form a 1.times.2 coupler, or two input fibers join two output fibers to form a 2.times.2 coupler. Previously, a number of these simple couplers were connected together in series to provide coupling between larger numbers of input and output fibers. More recently, monolithic 1.times.N and N.times.N couplers have been proposed to couple larger numbers of fibers.
A wide variety of monolithic coupler geometries have been proposed. These known monolithic optic couplers have generally been formed as tight bundles, in which each optical fiber is coupled to several surrounding optical fibers, or on occasion, to a central optical element in an axisymmetric arrangement. The interaction within these coupled structures can be quite complex, in part because the fused optical elements may form a waveguide which supports several propagation modes. Further complicating any analysis of these monolithic 1.times.N and N.times.N couplers is the real world existence of non-uniformity in the optical fiber bundle. The complex results of unequal fiber fusing in a simple triangular optic fiber bundle were analyzed by B. Michael Kale in "Performance of Lightly-Fused, Sharply-Tapered 3.times.3 Single Mode Fiber Optic Couplers," SPIE PROC., Vol. 839, Components For Fiber Optic Applications II, 1987, pp. 48-57. Where larger numbers of fibers are bundled together, analysis of these dimensional errors gets more complex, and the resulting non-uniformity in coupled power ratios may become more difficult to calculate and to avoid. Geometric distortion and the resulting inaccuracy in coupled power ratios appears to be difficult to eliminate within known bundled fiber geometries, particularly when the bundles of optical fibers are tightly bound within capillary tubes prior to tapering. Nonetheless, a wide variety of such bundled and clad monolithic fiber couplers have been proposed.
Another problem with many known optic couplers is that the coupled power ratios provided at the output tends to be wavelength dependent. Although the semiconductor lasers used to generate signals for modern optic networks are nominally coherent, the actual light signals produced are typically no more precise than .+-.30 nanometers of the nominal wavelength. To properly distribute these fairly noisy semiconductor laser generated signals, it is generally desirable that couplers distribute the signals properly regardless of the actual signal wavelength. In other words, it is generally preferable to provide couplers which are insensitive to wavelength variations. This helps to ensure that the strength of the signal on an arbitrary branch of a fiber optic network will be coupled properly.
While known 1.times.N and N.times.N optical couplers have been found to have advantages over couplers built up with several 1.times.2 or 2.times.2 devices, it is desirable to provide monolithic optical fiber couplers having improved wavelength response for high performance fiber optic networks. Furthermore, the described coupler fabrication techniques have proved to difficult to implement, and predictably and reliably reproducing desired coupled power ratios has proved to be particularly problematic. Without predictability, reproducibility, and manufacturability, the cost for these couplers has remained high, and implementation of fiber optic networks has thereby been inhibited.